1. Field of the Invention
This invention relates to an apparatus for measuring the gaseous content of industrial process gas streams, in a process in which a portion of the gas stream is extracted, conditioned for analysis and transported to a gas analysis device.
2. Description of Related Art
Due to more stringent Government regulations requiring that continuous emission monitoring systems demonstrate up-times approaching 100% in some cases, and the trend to require ever smaller operations to continuously monitor their gaseous emissions, a clear need exists for a highly reliable sampling and analysis system that will not cause an unreasonable financial burden on the smaller regulated operations.
Many gas analysis systems are not specific to one gas, so that gases other than the gas or gases of interest may cause an erroneous response in the analysis system. Dilution of the process gas stream with air to achieve lower concentrations of the interfering gas has been demonstrated as a reliable way to eliminate many interference problems.
One of the difficulties encountered in the certification process for continuous emissions monitoring systems is to obtain agreement between the system being certified and an independent reference monitoring system. Reference methods suggest frequent installation of clean gas filters, i.e., between 3 hour sample runs, to insure accurate results. However, it has been required by regulation that a continuous emission monitoring system operate for an extended period of time of at least 168 hours without unscheduled maintenance and provide accurate readings throughout the extended period. A continuous emission monitoring system can overcome this handicap by frequent calibration, injecting a known calibration gas through the same sample flow path as the process gas being measured, and correcting, either automatically or manually, for any gas loss across the sample filters and other sampling system components. However, no dilution probe currently used in the art allows for calibration through all of the filters in the sample flow path, and thus cannot account for the condition of the filters. The existing systems utilize a coarse or primary stack gas filter and a second or fine filter, and while these systems calibrate through the second filter, they do not calibrate through the coarse or primary filter.
A typical dilution probe 10 is shown in FIG. 1. All of the system components are contained inside a stainless steel pipe 12 which is inserted through stack wall 14 into the process environment. The dilution probe includes a coarse filter 16, a restrictor plate 18, a fine filter 20, a glass orifice 22, and an eductor assembly 24, all subject to process temperature extremes between 35.degree. and 580.degree. F. The apparatus operates with dilution air entering through port 26 producing a diluted gas stream 28 exiting the apparatus through port 30. Calibration gas enters the apparatus through port 32.
The flow stability of the critical components and the ultimate dilution ratio stability are extremely difficult to control under widely changing process conditions. Attempts have been made to control the temperature of the probe body to reduce the effects of changing process temperatures, but have met with limited success due to the wide range of possible temperatures in the highly corrosive environments of the process streams.
A further problem with the existing technology is the inability to calibrate through the first or coarse filter 16. Although this filter consists only of a stainless steel screen, in most cases substantial loss may be caused by this filter if the filter is allowed to become wet or partially plugged with a reactive particulate from the process.
This dilution probe is calibrated with calibration gas entering through port 32, and being pulled through the fine filter 20 into glass orifice 22, and then through dilution eductor 24. Restriction plate 18 is required to prevent the calibration gas from simply passing into the process stream; instead, the process gas flow is blocked and 100% calibration gas is provided to the inlet of the fine filter during calibration. However, any losses associated with filter screen 9 are not compensated during calibration, and while adequate for some of the cleaner processes, this technique is not acceptable for applications that have lower process temperatures or high moisture conditions usually associated with wet scrubbing pollution control systems.
Due to the configuration utilized in the existing dilution probe systems, blowback of the coarse filter element is also a problem. To clean the filter screen requires a rather explosive air burst, and given the restriction inherent in the present design, attempts to blowback the coarse filter have been difficult to accomplish.
Placement of the dilution eductor filtering system inside the stack is highly undesirable, since most processes attempt to operate above the acid dew point, i.e., above 300.degree. F., and many processes operate at temperatures between 280.degree. F. and 1200.degree. F. The prior art dilution probe shown in FIG. 1 cannot be used at temperatures above 580.degree. F. Extracting a four to eight foot stainless steel tube that weighs as much as 25 pounds from a process duct at a temperature of 580.degree. F. is not an east or safe maintenance procedure. Maintenance cannot be performed on the sampling system until the assembly nears ambient temperature and this may take several hours. Continuous emission monitoring systems that are required to obtain greater than 95% up-time could exceed the allowable down time requirements if any maintenance to the sampling system is required.
A dilution probe to be located primarily outside the stack is disclosed in U.S. Pat. No. 4,974,455. This probe includes a filter and associated heater located adjacent to the gas entrance port and an eductor and associated heater located adjacent to the sample exit port. The heaters are necessary to maintain the gas above its dew point as it passes through the apparatus. However, calibration of this apparatus is accomplished much in the same manner as in the previous apparatus, with a calibration gas introduced under pressure into the housing, forcing gas out through the coarse filter and back into the stack. In this process, the entire housing is purged of sample gas. Like the previous apparatus, this apparatus does not allow calibration through the coarse filter so that any gas lost through the coarse filter would not be measured.